Merge pull request #1751 from mrocklin/assumptions-context

New assumptions context manager
computations first commit
add edges
add CompositeComputation and unique function
inputs/outputs are attributes, not functions
actual tests. composite in_out solution
add reverse dict and toposort functions

Conflicts:

	sympy/utilities/iterables.py
	sympy/utilities/tests/test_iterables.py
add dag and toposort methods
add more complex test
add variables property
docstrings
__add__ and CompositeComputations can initialize
add str methods
remove composite testing class
add computation rules
add __eq__
add __eq__ for Composites
computations are Basics. Add Identity
top_down_once traveral applied to subs
remove Identity constructor. inputs/outputs LC